The present invention is directed to an optical cut-off filter which is formed of a multi-layer system comprising a sequence of dielectric layers which are interposed between two transparent media having the same index of refraction and the index of refraction of the dielectric layers is selected so that, for a preselected radiation angle of incidence, a pass band of the filter has an envelope of reflection for the secondary maxima for the s-component vibrating perpendicular to the radiation plane of incidence and an envelope for the p-component vibrating parallel to the radiation plane of incidence of the electrical field vector of the incident radiation, which envelopes exhibit zero locations.
An optical cut-off filter of this type has already been proposed. A cut-off filter is employed in optical communication technology as a mirror which is semi-transmissive for selected wavelengths and enables separating or combining different channels. The filters are so dimensioned that in the case of given oblique incidence of radiation, the filters for the wavelength of one channel exhibit at least an essentially disappearing reflectivity whereas for the wavelength of the other channel the filter has a reflectivity of nearly one.
The parameters of the proposed cut-off filter are so dimensioned that an envelope of the reflection-secondary maxima exhibits in the band pass of the filter at least virtually one zero location or position. For wavelengths in the vicinity of the zero location, the reflectivity is then at least virtually zero for the given radiation angle of incidence.
For specific multilayer systems, specific specifications for the dimensions can be disclosed. Such a specific multilayer system can also be so dimensioned that the envelopes of the reflection secondary maxima for radiation components vibrating perpendicular and parallel to the plane of incidence exhibit a common zero location. Such a specific cut-off filter is designated as being optimized. For wavelengths pertaining to the common zero location, the reflectivity is ideally zero for radiation components vibrating perpendicular to the radiation incident plane which components are the s-components as well as for radiation components vibrating parallel to the radiation incident plane which are designated p-components. For wavelengths in the approximate vicinity of the common zero location, the reflectivity for both components is virtually zero. In the case of the optimized cut-off filter, the channel spacing is fixed.